High impedance, low current faults, such as a downed distribution line conductor in an electric utility distribution network which is contacting a poor conductive earth composite, have proven to be difficult to isolate with present technology. Conventional overcurrent protection devices, both at the source and at strategic circuit locations, use the combined measurement of fault current magnitude and time duration to clear faults associated with downed grounded high voltage conductors. A particularly difficult situation for detecting a high impedance fault in an electrical distribution system involves a live conductor downed, but intact, and grounded through a poor conducting medium such as sand, rock, concrete, snow, blacktop or a tree. For reliability purposes, it is common practice to install downstream circuit reclosers, expulsion fuses or sectionalizers at all taps to the main stem distribution circuit. These protection devices serve to locally isolate downed fault conductors in the smallest sections possible, yet maintain normal service to the balance of customers on that same circuit. These downstream overcurrent protection devices are designed to be time coordinated with each other and a main circuit breaker in order to automatically isolate the downed primary conductor. Overcurrent protection devices are, however, unable to distinguish low fault currents (high impedance faults) from normal load currents because trip settings for these devices are typically set at 125 to 250 per cent of maximum estimated peak load current. These current levels are selected to minimize inadvertent tripping. A hazardous condition for the public is created when energized high voltage conductors fall to the ground or come in contact with a high impedance return path, and the overcurrent protection system fails to de-energize the conductor. Physical contact with an energized distribution primary conductor by any conducting body may cause serious injury or death due to electric shock. Numerous fatalities and serious injuries occur annually in the United States due to inadvertent contact with live down power distribution conductors. Experience has shown that these conditions occur more frequently at distribution level voltages of 15 KV and below, which is the predominant primary distribution voltage range in the United States.
Referring to FIG. 1, there is shown a simplified schematic diagram of a prior art high impedance fault sensing arrangement. An overhead distribution primary circuit 10 includes a substation bus 28 which is energized by a substation transformer 36 which is connected to the substation bus via a substation transformer breaker 34. Coupled to and extending from the substation bus 28 are plural branch tap circuits, each coupled to the substation bus by means of a respective distribution feeder breaker. Thus, two distribution feeder breakers are shown in FIG. 1 as elements 32a and 32b, with a third main overcurrent relay-circuit breaker combination shown as element 18 in the figure. The overhead distribution primary circuit 10 is subject to the occurrence of a low current, high impedance fault 12 on a branch tap 16 which is not detectable by a circuit recloser 14 or by the main overcurrent relay-circuit breaker combination 18. Branch tap 16, which is similar to other branch taps connected to distribution feeder breakers 32a and 32b, also includes plural distribution transformers 30a, 30b and 30c, and is shown experiencing the low current, high impedance fault 12, such as broken or downed conductor 29. A high impedance detection arrangement 20 coupled to the overhead distribution primary circuit 10 by means of a transducer 22 receives generated signals through the transducer. These signals are conditioned and compared by a microprocessor 24 with a stored signal pattern which is characteristic of normal system operation. A microcomputer 26 coupled to the microprocessor 24 as well as to the main overcurrent relay-circuit breaker combination 18 makes a trip/output decision based upon several operating parameters which are weighted. While the arrangement shown in FIG. 1 is designed for detection and shutdown of high voltage (low impedance) faults involving large currents, it is incapable of detecting and isolating low current high impedance faults. A low fault current isolator system is needed to permit electrical utilities to detect a high impedance fault characterized by a very low fault current to minimize the time period that a downed wire remains alive, after an overcurrent protection device has failed to de-energize the downed live wire.
The present invention overcomes the aforementioned limitations of the prior art by sensing the combination of loss of voltage on the load side of a downed conductor and live voltage on the source side of the downed wire. This downed wire constitutes a very high impedance fault characterized by a limited fault current typically below the tripping value of the associated fault isolating device. The detection, isolation and de-energization of the downed or damaged live conductor is analyzed and controlled by a host computer through remote tripping of an associated isolation device. This process occurs automatically and serves as a backup to a conventional overcurrent protection system for de-energizing high impedance electrical distribution primary faults, while permitting normal service to continue on the unaffected remainder of the power distribution circuit.
Plug-in, socket-type electric watthour meters are commonly used in the electric utility industry to measure electric power consumption at commercial, industrial or residential building locations. Meter installations typically include an enclosed panel, or cabinet, attached to an outside wall of the building structure which includes pairs of terminals connected to an electric power source, as well as to electric load conductors. The terminals are adapted to receive blade contacts of a plug-in electric meter to complete an electric circuit between the line and load terminals. Occasionally, it is necessary to disconnect electric power service to the building structure for various reasons. When this occurs, the electric meter is left in the cabinet to facilitate re-connection of the meter when electric service is restored. Various adapters have been developed to allow for mounting the meter in the electric power source cabinet while in an out-of-service status. One such adapter is disclosed in U.S. Pat. No. 4,311,354 which allows for positioning of the meter in various angular orientations for indicating that the meter is in an out-of-service condition. U.S. Pat. No. 5,033,973 discloses another out-of-service meter storage adapter which provides for a voltage potential applied to the meter source side while out-of-service for energizing the meter's semiconductor programmable memory and battery charging circuit. U.S. Pat. No. 8,177,580 discloses a disconnect adapter for an electric meter which enables the terminals of the meter socket to transfer only enough current to power the electric meter for monitoring and verifying that unauthorized electrical power is not being consumed.
There is a need for an out-of-service smart electric meter connection arrangement which allows the smart meter to continue operating in the electric power distribution system as by communicating with other smart meters, as well as communicating with a meter monitoring/control center, i.e., host computer, to facilitate the safe and convenient verification that unauthorized electric power is not being consumed at the site of the out-of-service meter while maintaining downstream communication in the smart meter system.
There is also the need for quickly and safely detecting, isolating and clearing a high impedance fault such as a downed electric utility primary distribution circuit conductor using a low cost, remotely controlled, spring-loaded grounding device.